Field of the Invention
The present invention relates to a crucible support heater for the control of a melt flow pattern within a crucible, and more specifically to controlling a temperature gradient or distribution during crystal growth by the Czochralski method through the use of a heater located in the crucible lift rod.
The well-known Czochralski method of growing crystals involves contacting a seed crystal with a melt of the crystal material contained in a heated crucible. The growth of large, substantially defect-free crystals by this and other methods remains to this day a difficult task, due to complications brought about by temperature gradients, convective eddies within the melt, and continuously changing thermal conditions.
Previous attempts at controlling the crystal growth process to improve the quality of the crystals produced have primarily involved controlling process parameters, such as, crucible or crystal rotation speeds, crystal pulling rate and total heat supplied to the crucible. The shape and stability of the solidification interface play an important part in achieving substantially defect-free large single crystals, and the shape and stability depend to a large extent on the flow patterns established in the melt. In practice, control of the crystal and crucible rotation rates have been used to attempt to control the flow pattern in the melt. The application of magnetic fields has also been used in attempting to suppress convective eddies within the melt to control the flow pattern, as well.
The above approaches share the disadvantage that the controls tend to produce only bulk effects within the melt. Control or manipulation of the crystal and crucible rotation rates without regard to the prevailing thermal boundary conditions, as has been the practice, leads to inconsistent results. Further, the use of magnetic fields does not guarantee a stable flow regime under the solidification interface.
Heaters disposed below the melt in containers from which single crystals are pulled have heretofore been disclosed. However, these heaters have been designed without the ability to assist in controlling the melt flow pattern in the melt container by selectively controlling and adjusting the thermal gradient across the entire bottom of the vessel. U.S. Pat. No. 4,654,110, issued to Morrison, discloses a shallow angle sheet crystal puller wherein heating rods disposed in the bottom of the melt-containing vessel have gradually increasing (or decreasing) heating capacity to provide a predetermined temperature gradient in the melt. However, there appears to be no means provided by which variation from the single predetermined gradient could be accomplished.
U.S. Pat. No. 4,659,421, issued to Jewett, discloses a shallow pan-type melt container having more than one heater disposed at the bottom of the container. However, these heaters are designed and used in a manner to minimize the effect on melt flow within the melt container. This Jewett patent discusses in detail the recognition of problems associated with melt flow in the Czochralski crystal pulling process. The invention in the Jewett patent is directed to a crystal pulling process which is quite different from the Czochralski method in that neither the crystal nor the melt container is rotated during the pulling process, and melt flow within the container is restricted by baffles, and is desirably kept to a minimum. The only desired melt flow using the apparatus and process in this patent is from below the feedstock area to below the crystal pulling area, and the melt flow is controlled either by the pumping action of the feed rod into the melt or by argon gas pressure in the region surrounding the feed rod. No suggestion is made in this patent to selectively control the thermal gradient across the bottom of a crucible to control the melt flow pattern to assist in controlling the shape of the solidification interface.
Exemplary of prior art which exhibits a control of the thermal gradient across a substantial portion of the bottom of the crucible is commonly assigned U.S. Pat. No. 5,162,072, to Azad. While the Azad patent has met with a degree of commercial success, the contact between the lift mechanism and the bottom of the crucible allows heat to be transferred from the bottom of the crucible to the lift mechanism. This heat loss causes the heater platform to raise the temperature of the bottom of the crucible in order to compensate for the heat loss. This additional heating by the platform heater on the bottom of the crucible may adversely affect the flow pattern of the melt contained within the crucible. Also, the platform heater may not provide the localized heating which is necessary to control the flow pattern of the central portion of the melt. Therefore, a more advantageous system, then, would be presented if the heat loss could be controlled while also controlling the thermal gradients across the bottom of the crucible.
It is apparent from the above that there exists a need in the art for a crystal growth apparatus which is capable of controlling the melt flow, and which at least equals the thermal gradient control characteristics of the crystal growth apparatus, particularly those of the highly advantageous type disclosed in the above-referenced Azad patent, but which at the same time is capable of reducing heat loss through the bottom of the crucible. It is a purpose of this invention to fulfill these and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.